KABUL, Afghanistan — Afghan police killed four American soldiers coming to their aid after a checkpoint attack Sunday, the third assault by government forces or insurgents disguised in military uniforms in as many days.

The escalating violence — including a NATO airstrike that killed eight Afghan women and girls gathering firewood — is straining the military partnership between Kabul and NATO as the U.S. begins to withdraw thousands of troops sent three years ago to route the Taliban from southern strongholds.

The attacks drew unusually strong criticism Sunday from the U.S. military’s top officer, Army Gen. Martin Dempsey, who called the problem of rogue Afghan soldiers and police turning their guns on allied troops “a very serious threat” to the war effort.

This year, 51 international service members have died at the hands of their Afghan allies or those who have infiltrated their ranks. At least 12 such attacks came in August alone, leaving 15 dead.

The surge in insider attacks is a sign of how security has deteriorated as NATO prepares its military exit from Afghanistan by the end of 2014. The U.S. is days away from completing the first stage of its own drawdown, withdrawing 33,000 troops that were part of a military surge three years ago. The U.S. will remain with about 68,000 troops at the end of September.

NATO and U.S. forces are working with the Afghan government to tighten vetting procedures and increase security between the forces, but nothing has so far been able to stem the attacks on troops, which NATO frequently asserts are standing “shoulder by shoulder.”

In unusually blunt remarks to the Pentagon’s own news service, the American Forces Press Service, Dempsey said the Afghan government needs to take the problem as seriously as do U.S. commanders and officials.

“We’re all seized with (the) problem,” said Dempsey, after discussing the issue at a meeting in Romania with NATO officials. “You can’t whitewash it. We can’t convince ourselves that we just have to work harder to get through it. Something has to change.”

“We have to get on top of this. It is a very serious threat to the campaign.”

A weekend of deadly attacks began Friday night, when 15 insurgents disguised in U.S. army uniforms killed two Marines, wounded nine other people and destroyed six Harrier fighter jets at a major U.S. base in the south, military officials said. On Saturday, a gunman in the uniform of a government-backed militia force shot dead two British soldiers in Helmand province in the southwest.

On Sunday, an Afghan police officer turned his gun on NATO troops at a remote checkpoint in the southern province of Zabul, killing four American service members, according to Afghan and international officials.

“It was my understanding that it was a checkpoint,” said Jamie Graybeal, a spokesman for international military in Afghanistan.

One police officer was killed in the clash with NATO troops, he said. Other officers at the site fled; it was unclear if they were involved in the attack or not.

Two international troops were wounded and were receiving treatment, Graybeal said. He did not say how serious the injuries were.

Afghan officials said the checkpoint in Zabul’s Mizan district came under attack first from insurgents sometime around midnight. American forces came to help the Afghan police respond to the attack, said Ghulam Gilani, the deputy police chief of the province.

International forces often work with Afghan police to man checkpoints as part of the effort to train and mentor the Afghan forces so that they can eventually operate on their own.

It was not clear if some of the Afghan police turned on the Americans in the middle of the battle, or were somehow forced into attacking the American troops by the insurgents, Gilani said.

“The checkpoint was attacked last night. Then the police started fighting with the Americans. Whether they attacked the Americans willingly we don’t know,” Gilani said.

He said all four of the dead were American, as did a U.S. official speaking on condition of anonymity because the information had not been officially released.

Dempsey said that the Friday assault on Camp Bastion, a British air base in Helmand province, should not be called an insider attack even though the attackers were wearing U.S. Army uniforms. An initial review found the attackers had no inside assistance, he said. It was not immediately clear how the attackers got the U.S. uniforms.

The latest deaths make at least 247 American troops killed in Afghanistan so far this year. Nearly 2,000 American troops have been killed in the conflict since the 2001 invasion.

Taliban spokesman Qari Yousef Ahmadi said the police who attacked were not affiliated with the Taliban insurgency.

“But they are Afghans and they know that Americans are our enemy,” Ahmadi told The Associated Press. He said in an emailed statement. He said the police who fled have joined up with the insurgency.

The coalition said in a statement that they were investigating what happened.

The airstrike that killed the eight women and girls, meanwhile, drew an apology from the U.S.-led coalition, condemnation from Afghan President Hamid Karzai and cries of “Death to America!” from villagers who retrieved the bodies.

Afghan officials said NATO planes killed eight women and girls in the remote Laghman province who had gone out before dawn to gather firewood.

The International Security Assistance Force, as the U.S.-led coalition is known, acknowledged that civilians had been killed and expressed its regret over the airstrike. It insisted known insurgents had been the target.

“ISAF takes full responsibility for this tragedy,” a statement said

Villagers from Laghman’s Alingar district drove the bodies, covered in white blankets, to the provincial capital, Mehterlam.

“They were shouting ‘Death to America!’ They were condemning the attack,” said Laghman provincial government spokesman Sarhadi Zewak.

Seven injured females were also brought to area hospitals for treatment, some of them as young as 10 years old, said provincial health director Latif Qayumi.

NATO forces spokesman Capt. Dan Einert said that the strike killed as many as 45 insurgents, but may have also killed five to eight Afghan civilians.

“Protecting Afghan lives is the cornerstone of our mission and it saddens us when we learn that our action might have unintentionally harmed civilians,” said Graybeal said.

Karzai “strongly condemns the airstrike by NATO forces which resulted in the deaths of eight women,” a statement from his office said. It said the Afghan government was also investigating.

The attacks come amid an international uproar about an Internet video mocking the Prophet Muhammad that many fear could further aggravate Afghan-U.S. relations. The video has sparked protests throughout the Muslim world and the Afghan government blocked the YouTube site that hosts the video and its parent company, Google Inc., over the weekend in a move to prevent violent protests.

The Taliban claimed that Friday’s assault on Camp Bastion was revenge for the video insulting Prophet Muhammad, but protests in Afghanistan have so far remained peaceful.

In the capital on Sunday, several hundred university students chanted “Death to America!” and “Long life to Islam!” over several hours to protest the video. Riot police cordoned off the area and the protest ended without incident in the early afternoon. A smaller protest went forward in the western city of Herat.

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